Systems and methods for using white light interferometry to measure undercut of a bi-layer structure

Abstract

Systems and methods for using white light interferometry to measure undercut of a bi-layer structure are provided. One such method involves performing a first scan of a first bi-layer structure with a microscope using a first scan range, where the microscope is configured for white light interferometry, generating a first interferogram using data from the first scan, performing a second scan of the first bi-layer structure with the microscope using a second scan range, generating a second interferogram using data from the second scan, determining a first distance between features of the first interferogram, determining a second distance between features of the second interferogram, and calculating a width of the undercut based on the first distance and the second distance. One such system involves using the microscope and/or a computer to perform one or more actions of this method.

What is claimed is: 
     
       1. A method for measuring an undercut of bi-layer structures on a wafer, the method comprising:
 performing a first scan of a first bi-layer structure having an upper layer disposed on a lower layer with a microscope using a first scan range corresponding to a first depth of scanning, wherein the microscope is configured for white light interferometry; 
 generating a first interferogram using data from the first scan; 
 performing a second scan of the first bi-layer structure with the microscope using a second scan range corresponding to a second depth of scanning different than the first depth of scanning; 
 generating a second interferogram using data from the second scan; 
 determining a first distance between first features of the first interferogram, the first features corresponding to respective physical edges of one of the upper layer or the lower layer of the bi-layer structure; 
 determining a second distance between second features of the second interferogram, the second features corresponding to respective physical edges of the other of the upper layer or the lower layer of the bi-layer structure; and 
 calculating a width of the undercut based on the first distance and the second distance. 
 
     
     
       2. The method of  claim 1 , wherein the first scan range is less than the second scan range, wherein the first and second ranges extend in a direction that is about normal to a top surface of the first bi-layer structure. 
     
     
       3. The method of  claim 2 :
 wherein the first features of the first interferogram comprise two interferogram edges each corresponding to a portion of the first interferogram having a maximum slope; and 
 wherein the second features of the second interferogram comprise two peaks each corresponding to a point of the second interferogram having a peak amplitude. 
 
     
     
       4. The method of  claim 3 :
 wherein an area of the lower layer is less than that of the upper layer; 
 wherein the two interferogram edges of the first interferogram correspond in position to outer physical edges of the upper layer; and 
 wherein the two peaks of the second interferogram correspond in position to outer physical edges of the lower layer. 
 
     
     
       5. The method of  claim 3 :
 wherein an area of the lower layer is less than that of the upper layer; 
 wherein the first bi-layer structure comprises a centrally disposed hole extending through both the upper layer and the lower layer; 
 wherein the two interferogram edges of the first interferogram correspond in position to the physical edges of the upper layer defining the hole; and 
 wherein the two peaks of the second interferogram correspond in position to the physical edges of the lower layer defining the hole. 
 
     
     
       6. The method of  claim 2 :
 wherein the first features of the first interferogram comprise four interferogram edges each corresponding to a portion of the first interferogram having a maximum slope; 
 wherein the second features of the second interferogram comprise four peaks each corresponding to a point of the second interferogram having a peak amplitude; 
 wherein the performing the first scan of the first bi-layer structure with the microscope using the first scan range comprises performing the first scan of the first bi-layer structure and a second bi-layer structure with the microscope using the first scan range; and 
 wherein the performing the second scan of the first bi-layer structure with the microscope using the second scan range comprises performing the second scan of the first bi-layer structure and the second bi-layer structure with the microscope using the second scan range. 
 
     
     
       7. The method of  claim 6 :
 wherein an area of the lower layer of the first bi-layer structure is less than that of the upper layer of the first bi-layer structure; 
 wherein the second bi-layer structure comprises an upper layer on a lower layer, where an area of the lower layer of the second bi-layer structure is less than that of the upper layer of the second bi-layer structure; 
 wherein a first two interferogram edges of the four interferogram edges of the first interferogram correspond in position to outer physical edges of the upper layer of the first bi-layer structure; 
 wherein a second two interferogram edges of the four interferogram edges of the first interferogram correspond in position to outer physical edges of the upper layer of the second bi-layer structure; 
 wherein a first two peaks of the four peaks of the second interferogram correspond in position to outer physical edges of the lower layer of the first bi-layer structure; and 
 wherein a second two peaks of the four peaks of the second interferogram correspond in position to outer physical edges of the lower layer of the second bi-layer structure. 
 
     
     
       8. The method of  claim 2 :
 wherein the first features of the first interferogram comprise four interferogram edges each corresponding to a portion of the first interferogram having a maximum slope; 
 wherein the second features of the second interferogram comprise four peaks each corresponding to a point of the second interferogram having a peak amplitude; 
 wherein an area of the lower layer of the first bi-layer structure is less than that of the upper layer of the first bi-layer structure; 
 wherein the first bi-layer structure comprises a first hole and a second hole, each extending through both the upper layer and the lower layer; 
 wherein a first two interferogram edges of the four interferogram edges of the first interferogram correspond in position to a first two physical edges of the upper layer of the first bi-layer structure defining the first hole; 
 wherein a second two interferogram edges of the four interferogram edges of the first interferogram correspond in position to a second two physical edges of the upper layer of the first bi-layer structure defining the second hole; 
 wherein a first two peaks of the four peaks of the second interferogram correspond in position to a first two physical edges of the lower layer of the first bi-layer structure defining the first hole; and 
 wherein a second two peaks of the four peaks of the second interferogram correspond in position to a second two physical edges of the lower layer of the first bi-layer structure defining the second hole. 
 
     
     
       9. The method of  claim 1 , wherein the second scan range is less than the first scan range, wherein the first and second ranges extend in a direction that is about normal to a top surface of the first bi-layer structure. 
     
     
       10. The method of  claim 1 , further comprising modifying a fabrication process for forming the first bi-layer structure if the width of the undercut is outside of a preselected range. 
     
     
       11. The method of  claim 10 , wherein the modifying the fabrication process comprises adjusting a duration of a development sub-process used in forming the undercut of the first bi-layer structure. 
     
     
       12. The method of  claim 1 :
 wherein the performing the first scan of the first bi-layer structure with the microscope using the first scan range comprises:
 providing the wafer; 
 depositing polymethylglutarimide (PMGI) on the wafer to form the lower layer; 
 depositing photo resist on the lower layer to form the upper layer; 
 patterning the lower layer and the upper layer, where the patterning comprises developing the lower layer and the upper layer to form the first bi-layer structure; and 
 performing the first scan of the first bi-layer structure with the microscope using the first scan range.